Search results for "Quantum Fluctuation"
showing 10 items of 92 documents
Probing the creatable character of perturbed Friedmann-Robertson-Walker universes
2008
We discuss whether some perturbed Friedmann-Robertson-Walker (FRW) universes could be creatable, i. e., could have vanishing energy, linear momentum and angular momentum, as it could be expectable if the Universe arose as a quantum fluctuation. On account of previous results, the background is assumed to be either closed (with very small curvature) or flat. In the first case, fully arbitrary linear perturbations are considered; whereas in the flat case, it is assumed the existence of: (i) inflationary scalar perturbations, that is to say, Gaussian adiabatic scalar perturbations having an spectrum close to the Harrison-Zel'dovich one, and (ii) arbitrary tensor perturbations. We conclude that…
Nonlinear relaxation phenomena in metastable condensed matter systems
2016
Nonlinear relaxation phenomena in three different systems of condensed matter are investigated. (i) First, the phase dynamics in Josephson junctions is analyzed. Specifically, a superconductor-graphene-superconductor (SGS) system exhibits quantum metastable states, and the average escape time from these metastable states in the presence of Gaussian and correlated fluctuations is calculated, accounting for variations in the the noise source intensity and the bias frequency. Moreover, the transient dynamics of a long-overlap Josephson junction (JJ) subject to thermal fluctuations and non-Gaussian noise sources is investigated. Noise induced phenomena are observed, such as the noise enhanced s…
Dielectric relaxation of space charges and polarons in ferroelectric perovskites
2001
Abstract In all ferroelectric perovskites, intentionally introduced or “unwanted” point defects do play a role in the dielectric spectra and in the conductivity. Above room temperature, space charge relaxation at the electrodes interfaces are observed. This can be of interest in the context of the nowadays applications of ferroelectric thin films. At liquid helium temperatures much more localised dielectric relaxation occurs. Special emphasis will be brought on SrTiO3 which has received renewed interest at the beginning of the nineties and for which a wealth of reliable experimental data are available. Considering that a gradual freezing of polarized objects is occurring at low temperatures…
Evidence of quantum phase slip effect in titanium nanowires
2012
Electron transport properties of titanium nanowires were experimentally studied. Below the effective diameter $\lesssim$ 50 nm all samples demonstrated a pronounced broadening of the $R(T)$ dependencies, which cannot be accounted for thermal flcutuations. An extensive microscopic and elemental analysis indicates the absence of structural or/and geometrical imperfection capable to broaden the the $R(T)$ transition to such an extent. We associate the effect with quantum flucutuations of the order parameter.
Time evolution of linearized gauge field fluctuations on a real-time lattice
2016
Classical real-time lattice simulations play an important role in understanding non-equilibrium phenomena in gauge theories and are used in particular to model the prethermal evolution of heavy-ion collisions. Due to instabilities, small quantum fluctuations on top of the classical background may significantly affect the dynamics of the system. In this paper we argue for the need for a numerical calculation of a system of classical gauge fields and small linearized fluctuations in a way that keeps the separation between the two manifest. We derive and test an explicit algorithm to solve these equations on the lattice, maintaining gauge invariance and Gauss's law.
Emittance Growth by Synchrotron Radiation in a Double-Sided Microtron
1999
Here we present results of calculations of emittance growth caused by quantum fluctuations of synchrotron radiation (QFSR) for a 1.5 GeV double-sided microtron (DSM). We did both semi-analytical estimations, employing known Twiss parameters for the DSM orbits, and a computer simulation of these stochastic effects using the program SYTRACE. This showed that the normalized emittance growth was within reasonable limits, by a factor of about 1.5, thus permitting e.g. the installation of small aperture linacs on the DSM axes.
Quantum effects and orientational ordering in adsorbed layers of linear molecules
1994
We study the influence of quantum fluctuations on the herringbone transition in adsorbed complete √3-mono-layers of diatomic molecules. Using Path-Integral Monte Carlo simulations for rotations, we can quantify the shift of the transition temperature for a highly realistic model to describe N2 on graphite. In addition, the zero-point motion of the librating molecules depresses the ground-state order parameter. We compare the benchmark data to quadratic Feynman-Hibbs effective potential simulations and to a quasiharmonic approximation. Using a simplified model for this transition, we study systematically quantum effects being relevant for lighter molecules. Depending on the rotator's rotatio…
The Ferroelectric Photo-Groundstate of SrTiO$_3$: Cavity Materials Engineering
2021
Significance Controlling collective phenomena in quantum materials is a promising route toward engineering material properties on demand. Strong THz lasers have been successful at inducing ferroelectricity in S r T i O 3 . Here we demonstrate, from atomistic calculations, that cavity quantum vacuum fluctuations induce a change in the collective phase of S r T i O 3 in the strong light–matter coupling regime. Under these conditions, the ferroelectric phase is stabilized as the ground state, instead of the quantum paraelectric one. We conceptualize this light–matter hybrid state as a material photo ground state: Fundamental properties such as crystal structure, phonon frequencies, and the col…
The quantum paraelectric phase of SrTiO$_3$ from first principles
2021
We demonstrate how the quantum paraelectric ground state of SrTiO$_3$ can be accessed via a microscopic $ab~initio$ approach based on density functional theory. At low temperature the quantum fluctuations are strong enough to stabilize the paraelectric phase even though a classical description would predict a ferroelectric phase. We find that accounting for quantum fluctuations of the lattice and for the strong coupling between the ferroelectric soft mode and lattice elongation is necessary to achieve quantitative agreement with experimental frequency of the ferroelectric soft mode. The temperature dependent properties in SrTiO$_3$ are also well captured by the present microscopic framework.
Quantum Creep and Quantum-Creep Transitions in 1D Sine-Gordon Chains
2003
Discrete sine-Gordon (SG) chains are studied with path-integral molecular dynamics. Chains commensurate with the substrate show the transition from collective quantum creep to pinning at bead masses slightly larger than those predicted from the continuous SG model. Within the creep regime, a field-driven transition from creep to complete depinning is identified. The effects of disorder in the external potential on the chain's dynamics depend on the potential's roughness exponent $H$, i.e., quantum and classical fluctuations affect the current self-correlation functions differently for $H = 1/2$.